Drugs that selectively block cell survival pathways in malignant cells may lead to improved therapeutic outcomes. Our previous studies have identified a tyrosine kinase-independent signalling mechanism that regulates cell survival but not other cellular responses such as proliferation. We refer to this ability of some cells to survive in a largely quiescent (or dormant) state as the “survival-only” response and have shown that constitutive activation of survival-only pathways renders quiescent acute myeloid leukemia (AML) cells highly refractory to cytotoxic therapies. Thus, identifying therapeutic targets within survival-only pathways may provide new clinical approaches in the treatment of AML and possibly other cancers. We have performed both biochemical and functional screens using kinase inhibitors and siRNA to identify kinases that regulate survival-only responses in leukemic cells. These screens identified 7 “survival kinases” (Pim2, Cdk4, Cdk9, PKCϵ, AurK, CK2 and PI3K) that when subjected to siRNA knockdown in primary human AML cells resulted in a significant reduction in cell survival (p<0.05). Furthermore, dual siRNA knockdown of Pim2/CK2, Pim2/PKCϵ, Cdk4/AurK, PI3K/Cdk9 resulted in a further significant reduction in primary human AML cell survival compared to single knockdowns (p<0.05). We therefore sought to identify dual kinase inhibitors with the ability to simultaneously block 2 of the 7 survival kinases identified in our screens. Taking a panel of known Pim2, Cdk4, Cdk9, PKCϵ, AurK, CK2 and PI3K inhibitors, we examined each for the ability to inhibit their known “on-target” kinase as well as their ability to inhibit any of the remaining 6 “off-target” kinases using either in vitro (inhibition of recombinant kinases) or cellular (phosphospecific Abs in Western blots) assays. We have identified 6 compounds that are able to inhibit the kinase pairs, PI3K:CK2, PKC:Pim2, Pim2:Cdk9, AurK:Cdk4, PI3K:Cdk9 and PIM2:CK2. Importantly, 5/6 compounds identified in our screens induced significant apoptosis in primary human AML cells (>70% apoptosis) but not in purified human CD34+ bone marrow progenitors from normal donors (p<0.05). Our findings provide a novel and rational approach for identifying previously unsuspected vulnerabilities in cancer cells through the dual inhibition of distinct and independent survival pathways. They also reveal previously unrecognized alliances between evolutionarily divergent kinases in promoting tyrosine kinase-independent survival-only responses and show that inhibition of multiple survival arms in AML cells is highly synergistic.

Citation Format: Daniel Thomas, Jason A. Powell, Michael W. Parker, Paul G. Ekert, Richard B. Lock, David CS Huang, Susie K. Nilsson, Christian Recher, Andrew H. Wei, Mark A. Guthridge. The selective targeting of cell survival pathways in leukemia. [abstract]. In: Proceedings of the Third AACR International Conference on Frontiers in Basic Cancer Research; Sep 18-22, 2013; National Harbor, MD. Philadelphia (PA): AACR; Cancer Res 2013;73(19 Suppl):Abstract nr A19.